Patent GB-664496-A (GB)
Title: Improvements in and relating to electronic control systems
IPC: B65H23/032
Filed: 1948-02-27
Abstract: 664,496. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 27, 1948. [Feb. 21, 1947] No. 6118/48. Class 38 (iii). [Also in Groups XXXV and XL (b)] Inverse variation of the grid voltages of a pair of electric valves having separate anode impedances and a common cathode impedance, produces variation of the relative energisation of a pair of control circuits, each connected from a point of relatively high voltage of a different one of the anode impedances to a point of lower voltage of the other, and thereby controls a dynamo-electric machine, In Figures 1a, 1b a length of material 1, e.g. paper or steel, being unwound from a braked reel 2 is automatically maintained by a D.C. motor 4 in such a position laterally that approximately half the light from each of two lamps 11, 12 falls on photocells 9, 10 respectively. Photocells 9, 10 are each associated with a different grid circuit of a double-triode valve 26 which, like other valves in the control circuit, receives its anode supply from an A.C. source 14 through a full-wave rectifier 13. Variable resistors 32, 34, 39 in the grid circuits of valve 26 are initially adjusted so that the anode currents of valve 26 are equal both when no light falls on the photocells and when light is falling on them with the material in its desired lateral position. Under such conditions the potentials of points 40a, 42a and hence, through lines 54, 55, the potentials of the grids of a double-triode valve 48 are equal so that two circuits between points 50a, 53a and 51a, 52a in the anode circuits of valve 48 carry equal currents. Included in these two circuits are the D.C. windings 59a, 62a of saturable reactors 59, 62, which are thus equally energised so that in-phase A.C. potentials are applied from a transformer 67 to the grids of thyratron valves 64, 65 reversely connected in series with motor 4 across A.C. source 14 and motor 4 is stationary. A switch 30 is so positioned that deviation of the material from its correct lateral ,position differentially varies the saturation of reactors 59, 62 in such a sense that the resultant pulsating D.C. supplied through thryatrons 64, 65 to motor 4 causes it to run in the appropriate direction to correct the lateral position of the material. Diodes 60, 63 ensure that current can pass in only one direction through saturating windings 59a, 62a. To limit the pulsating current in motor 4 the output of a current transformer 70 is rectified by the double diode of a double-diodetriode valve 71 and when the motor current exceeds a predetermined value the voltage across a condenser 69, which is normally maintained constant through the diode-connected triode of valve 71, rises to render conducting a triode valve 72 which increases the bias of valve 48 and prevents further increase of motor current. To prevent hunting a portion of the output of one triode of valve 26 is applied through a potentiometer 76 to the grid of a triode valve 73 from the anode circuit of which is derived through condensers 77, 78, a rate of deviation signal which is injected through line 55 into the grid circuit of one triode of valve 48. Sense of deviation is indicated by an electron indicator tube 79.